The National Virtual Observatory
2006 Summer School

Getting down and dirty with VO tools.

Tom McGlynn (NASA/GSFC)

The purpose of this talk get everyone some hands on experience with a few tools that have not been part of exercises so far. Only a few features of each will be illustrated but you can follow up at your leisure.

Can we crash DataScope?

DataScope is a powerful tool for exploring the datasets available in the VO. Let's just see how well it works with forty users...
  1. Go the the NVO Home page
  2. I will start a new DataScope request, to look for data on Abell 1656.
  3. When prompted enter 'Abell 1656' in the second entry box on the NVO Home page, under "Discover and Explore Data in the Virtual Observatory from archives and data centers around the world."
  4. If we get reasonable response, you will see your page update dynamically as new results are obtained.
  5. Click on the tabs to see what each holds.
  6. Within the Resources Tab, click on the "CADC/HST" link not the checkbox before it). You'll be sent into the Data Table tab and shown the results from this SIAP Service. Check a few buttons at the beginning of some of the rows.
  7. Click on the Resource tab again to get back there, and then click on the SDSS DR5 link.
  8. Click on a few of the checkboxes there.
  9. Back to Resources...
  10. Below the SIAP Services there are lists of catalogs and objects. This are cone services. Try clicking on some of the checkboxes on this page.
  11. Click on the Summary Tab. It should give the count of the number of FITS files from SIAP services that you have selected, and the number of XML files from either Cone or SIAP services. You can save these files or send them to Aladin or OASIS. Try that at some more leisurely moment.
  12. Take a quick gander at the other tabs.

Viewing images and catalogs in Aladin

Aladin is a very powerful tool for comparing images and catalogs with many, many facilities for data extraction and display. This execise just scratches the surface.
  1. Make sure you've run the NVOSS setup.
  2. cd $NVOSS_HOME/java/data
    We have just one FITS and a few catalog files here. Rather than jam the network we'll use these to start up...
  3. aladin ../data/abell.xml ../data/abel254.fits The Aladin command we supply does a CD to the lib directory and then executes Aladin from there. That's why we need to put the directory prefixes for the files.
  4. You may be requested to upgrade Aladin. Feel free to do so later.
  5. You should get the Aladin user interface. There is an eye icon on the right which looks the the input planes. You should get two input planes, one for each file.
  6. Click repeately on the table icon to the left of the 'abell.xml'. You should see a little mark appear and disappear in the center of the image. When it appear click on it. The tabular information for that icon will appear at the bottom.
  7. Click on the Load button. A new window showing the myriad data sources you can use in Aladin will appear.
  8. Aladin's power is really shown with multiple images and data. At home try go back to the Abell 1656 search in DataScope and pick the first Chandra image, the FIRST image, and one of the DSS images (remember to select the FITS images, not the XML files returned). Make a three color image that shows the smooth X-ray background in the cluster superposed on the optical galaxies and also shows a classic two-lobed radio source just to the right of center.

    Building images with SkyView

    SkyView is a tool which automatically converts survey and other images to the geometry the user wants from whatever the original geometry may have been. There is a complete distribution of SkyView in the SSS. If you want to generate hundreds or thousands of images with a specified geometry from any of the major input surveys -- or with just a little work from any appropriate SIAP service -- this may be a useful tool. SkyView is supplied as a self-contained JAR file with both software and data. The source code and documentation is also included in the distribution.
    1. cd $NVOSS_HOME/java/src/skyview
    2. Let's just look at what the input options are
      java -jar skyview.jar
    3. There are a couple of test files in the directory. The sdssi.test downloads quite a bit of data, so it may clog things up. Try it when there won't be 40 simultaneous users! If we look at heao1.test we see: 
      java -jar skyview.jar survey=heao1a position=0,0 coordinates=galactic \
          pixels=800,400 size=360,180 projection=Car \
          lut=fire catalog=uhuru4 \
          output=heao1 quicklook=gif
      This generates an all-sky Cartesian X-ray map in galactic coordinates. It will create both a FITS and GIF image.
    4. The test files are not executable, so try: csh < heao1.test

      Windows users will need to do a little more work since the backslashes don't seem to be recognized. You might try the sdssi.test instead (e.g., cmd < sdssi.test ). There are few enough Windows users that that should be OK.

    5. When your command finishes you should see both a FITS (heao1.fits or sdssi.fits) and quicklook (heao1.gif or sdssi.jpg) image. Pop the quicklook into your browser.
    6. Take a look at the skyviewinajar.doc (Word) file for more information at your leisure.

    TOPCAT ploting

    While we've used TOPCAT to combine tables, it also has some powerful display capabilities that we haven't shown. Since TOPCAT cannot be run as an applet, we often choose VOPlot for plotting, but let's just take a quick gander at TOPCAT's which are often more convenient if we can run as an application.
    1. topcat
    2. Once the interface appears click on File/Load Table
    3. Load in the intermediate data from Brian's talk: http://chart.stsci.edu/twiki/pub/Main/NVOSS3CourseNotes/scienceApp3_table1.xml
    4. Click on the globe icon to see the positions on the sky. Try dragging the globe around.
    5. On the main page click on the scatterplot icon (right after the histograms). Select a and b as the plot axes.
    6. Click on "Subsets/Draw region" from the menu at the top of the plotting window.
    7. Draw a region to enclose a few of the points on the plot.
    8. Click on "Subsets/Finish Drawing region". Note the region identifier that shows up on the bottom right.
    9. Change the plot to an RA/Dec plot. Note how we follow the region selected in one domain in plots made in another domain.
    Again this is just a taste of what you can do with TOPCAT plotting. >